In the new theory proposed in 1923 independently by Brønsted and Lowry, acids are defined as molecules or ions that produce H+ ions and molecules or which received H + is a partner of the acid base.
Base are not just a molecule or ion that produces OH-, but which received H+. Because the acid HA, H+ to produce water in solution in water and produce oksonium ion, H3O+, water is also a base according to this definition.
HA (acid) + H2O (base) → H3O+ (conjugate acid) + A-(conjugate base)
Here, H3O+ is the conjugate acid and conjugate base is A-. However, since water also provides H+ into ammonia and produce NH4+, the water is too acidic, as shown in the following equation:
H2O (acid) + NH3 (base) → NH4 + (conjugate acid) + OH-(conjugate base)
So water can be either acidic or alkaline depending co-reactant. Although the Bronsted Lowry definition is not too different from the Arrhenius definition, the definition is broader benefits as it can be used to acid-base systems in non-aqueous solvent.